Titanium nitride is used in a variety of applications for integrated circuit fabrication. It has been used for an adhesion layer for tungsten film, as a local interconnect and as a diffusion barrier.
As an adhesion layer, titanium nitride offers advantages in applications where tungsten is used for contact hole and via filling. The process is normally started by depositing a thin layer of a material that acts to improve adhesion between the tungsten and underlying dielectric. Since tungsten adheres poorly to dielectric materials, a material must be used which adheres well to the dielectric and then adheres well to the tungsten. Titanium nitride is such a material and provides several advantageous properties such as very low resistivity and a resistance to the chemistries used to etch tungsten, as well as exhibiting good adhesion to both dielectric and tungsten films.
Titanium nitride can also be used as a barrier layer because is serves as an impermeable barrier to silicon. It also has an activation energy higher than other materials. For example, the activation energy for copper diffusion into titanium nitride is reported to be 4.3 electron volts, while the activation energy from copper into most metals is on the order of 1 to 2 electron volts.
Typically, titanium nitride is formed by vapor evaporation of titanium in a nitrogen ambient, by reactively sputtering titanium in a nitrogen/argon mixture, by sputtering titanium nitride from a target in an argon environment, by depositing titanium and then converting it to titanium nitride in a subsequent nitridization step, or by the thermal chemical vapor deposition reaction employing titanium tetrachloride and ammonia, or metal-organic precursors.
There are limitations to many of these techniques. In the case of sputtered films, poor conformality is a problem at high aspect ratio vias. In the case of thermal chemical vapor deposition films, the conformality over high aspect ratio geometries is excellent. However, the high deposition temperatures employed make chemical vapor deposition of titanium nitride impractical for use in multi-level metalization schemes. Temperatures greater than 400.degree. C. cause unwanted thermal stress to aluminum layers resulting in hillock formation and damage to the inter-metal dielectric layers.